The invention relates to a valve sleeve which forms an oil rotary valve and a method of manufacturing same.
An oil rotary valve as used in a power steering apparatus, for example, comprises a valve rotor in which a plurality of axial grooves are formed, and a valve sleeve which is rotatably fitted around the valve rotor and formed with axial grooves in its inner surface which can be aligned with the circumferentially opposite sides of a groove in the valve rotor. In response to a relative rotational displacement between the valve rotor and the valve sleeve, the supply and discharge of hydraulic fluid to or from a power cylinder can be controlled. The grooves formed in the valve sleeve are defined as blind grooves, namely, a groove in which only its middle portion is recessed to serve as a groove while the opposite ends must not be recessed.
A technique which forms such blind groove in the inner surface of a cylindrical valve sleeve is disclosed in Japanese Patent Publication No. 49,541/1974. In this technique, a cutter of a given size is repeatedly driven back and forth along an arcuate path of movement into the inside of a cylindrical material to form a single groove. A plurality of grooves are formed successively by synchronously rotating the cylindrical material. However, the described technique suffers from disadvantages that an apparatus having a complex construction is required and an increased length of working time is necessary.